Fix disclaimer text not centralizing when maximized (#12374)

## Summary of the Pull Request
Fix the disclaimer text boxes in `Rendering` and `Defaults`not centralizing along with the expanders when the window is maximized

## PR Checklist
* [ ] Closes #xxx
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [ ] Schema updated.
* [x] I work here

## Validation Steps Performed
<img width="1128" alt="defaults" src="https://user-images.githubusercontent.com/26824113/152584084-a999cb29-73bc-4970-889a-f95ea64c1b4c.png">
<img width="1128" alt="rendering" src="https://user-images.githubusercontent.com/26824113/152584099-a54519da-7bca-4ebe-b487-b68ac5cf1a37.png">

src/cascadia/TerminalSettingsEditor/CommonResources.xaml

@@ -66,6 +66,7 @@
            TargetType="TextBlock">
         <Setter Property="FontStyle" Value="Italic" />
         <Setter Property="TextWrapping" Value="WrapWholeWords" />
+        <Setter Property="MaxWidth" Value="1000" />
     </Style>
 
     <!--  Used for flyout messages  -->